Lab 3: Experiment 2

Input Expressions

To make some of the other experiments easier, we'll take a look at input expressions.

Variables Only

Consider this statement:

cin >> 3;

What do you suppose this would mean? Maybe after this executes, every 3 the program encounters should be replaced with the value the user types in from the keyboard? That seems awfully silly and quite dangerous. So, a better question: does C++ even allow this? Try it. Add the input statement to your program and recompile.

Question #3.2.1: What is the first compiler error that you get?

Fortunately, C++ doesn't let us do something so silly.

Instead, all of the objects in an input expression must be variables. Let's try this:

cin >> i >> j;

Add this statement after the declaration of i and j, but before the output statement. When you run your program, enter these values:

123  456

Question #3.2.2: What values are printed for i and j? Did these values come from the declaration or from your keyboard input?

An input statement replaces the value in a variable used in the input statement. The variable does not remember its old values; the old value is gone.

Replacing Values

Try this variation:

Question #3.2.3: What happens if the input line is moved before the declaration?

That's actually a review of the previous lab. Remember that you can't use a variable unless you declare it first. The order of the statements matter.

Here's a variation that will compile and execute:

Question #3.2.4: What happens if the input line is moved after the output statement?

Question #3.2.5: Use the results of this experiment to justify the claim that an input statement replaces the values in its variables.

Question #3.2.6: Is it necessary to initialize i and j in their declaration if we read in values for them in the very next statement?

Wrapping Up

Move the input statement back between the declaration and output statements.

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